The present invention relates to fine copper powder and a process for producing the same. More particularly, it relates to fine copper powder which exhibits suitable characteristics when used in an electrically conductive paste or an electrically conductive adhesive and an economical and convenient process for producing the same.
In the field of electronic components mounting techniques for OA equipment, portable communication equipment, etc., electrically conductive pastes recently used in chips, contact bump materials, and the like include those comprising a resin binder, glass frit, etc. having incorporated therein conductive metal particles or flakes mainly comprising silver or silver-palladium. These conductive pastes are also used for through-holes, cross-overs, electrodes, etc. of printed wiring boards. The conductive pastes containing conductive metal powder or flakes mainly comprising silver or silver-palladium, while excellent in electrical conductivity and resistance to oxidation, are disadvantageous in that such a metal powder as silver, palladium, etc. is expensive and difficult to obtain stably and has a problem of migration resistance. Hence the need for copper powder that is cheap and excellent in electrical conductivity has been increasing as a substitute for expensive silver or palladium.
Currently adopted processes of producing copper powder include atomizing, electrolysis, wet synthesis, and the like. Copper powder obtained by atomizing or electrolysis, which is chiefly used in powder metallurgy, has an average particle size of about several tens of microns. On the other hand, wet synthesis provides copper powder having an average particle size regulated between about 0.2 to 4 xcexcm with a narrow particle size distribution but involves high cost and has an economical problem.
With the electronic equipment and the like having been reduced in size and weight, the pitch of the conductive circuits therefor has been made finer. To cope with this trend, the copper powder to be used in conductive pastes for through-holes of printed wiring boards has been required to be finer, specifically to have an average particle size of 10 xcexcm or smaller, preferably an average particle size between about 3 and 5 xcexcm. As mentioned above, although wet synthesis furnishes copper powder having such a range of average particle size, it is economically disadvantageous and cannot be said to be an industrial method of manufacture. The copper powder obtained by atomizing generally has an average particle size of several tens of microns as described above. If such a copper powder is classified to obtain particles of 10 xcexcm or smaller, the yield is poor, and an increase in cost results.
To meet the above demand, copper powder having an average particle size of about 8 xcexcm has been obtained by grinding electrolytic copper powder having an average particle size of about 20 to 35 xcexcm by means of an atomizer. However; still finer copper powder has been demanded for use in conductive pastes. Use of a high water pressure atomizer could provide copper powder having an average particle size of about 5 xcexcm, but the production yield is poor, which is economically disadvantageous.
Japanese Patent Application Laid-Open Nos. 199705/87 and 182809/90 disclose crushing and pulverizing electrolytic copper powder by collision among copper particles give a fine copper powder of 10 xcexcm or smaller in average particle size, that is, a method of crushing and pulverizing electrolytic copper by use of a jet mill of the system in which particles are made to collide with each other. This method, however, involves the problem that a conductive paste using the resulting fine copper powder is inferior in characteristics such as electrical conductivity.
It is possible to obtain fine copper powder of 3 to 5 xcexcm in average particle size by pulverizing electrolytic copper powder by means of a jet mill of collision plate type. However, the fine copper powder obtained by this method comprises granular particles or a mixture of granular particles and twiggy particles. It is desired for the fine copper powder used in a conductive paste to comprise not only granular particles, etc. but flaky or flat particles from the standpoint of prevention of sagging and improvement of electrical conductivity.
Accordingly, an object of the present invention is to provide fine copper powder having suitable characteristics for use in conductive pastes and conductive adhesives, particularly satisfactory electrical conductivity and anti-sagging effects, and an economical and convenient process for producing the same.
As a result of investigations, the present inventors have found that the above object is accomplished by pulverizing dendritic electrolytic copper powder having specific properties and having been coated with fat and oil by means of a jet mill of collision plate type into fine powder or by flattening fine granular copper powder by means of a medium type agitation mill.
The present invention has been completed based on the above findings and provides a process for producing fine copper powder characterized by comprising mixing fat and oil into a dendritic electrolytic copper powder having an average particle size of 20 to 35 xcexcm and a bulk density of 0.5 to 0.8 g/cm3 to coat the surface of the electrolytic copper powder particles with the fat and oil and pulverizing the coated particles by means of a jet mill of collision plate type.
The present invention also provides a fine flaky copper powder characterized by having an average major axis diameter of 4 to 10 xcexcm and a flakiness of 2 to 20.
The present invention also provides a preferred process for producing the above-mentioned fine flaky copper powder, which is characterized by comprising introducing a copper slurry of fine granular copper powder having an average particle size of 3 to 5 xcexcm dispersed in water into a medium type agitation mill and flattening the fine copper powder.